Recently, a television system is becoming larger in size and higher in picture quality, and a scan converter is developed for converting interlaced scan video signals of present broadcast into progressive scan video signals to be displayed.
As such a scan converting method, a prior art is described in the following. Input video signal is compared with fields or frames before and after the signal, and detected whether it is of moving picture region, still picture region, or medium region in the middle of the both regions, and classified into these three kinds of regions. In the still picture region, the input video signal and video signals before and after the signal are synthesized as odd horizontal scanning lines and even horizontal scanning lines, thereby executing line interpolation, and then converted into progressive scan video signal. In the moving picture region, the input video signal and video signals of horizontal scanning lines above and under the signal are used to obtain video signals of horizontal scanning lines to be interpolated. In the medium region, line interpolation is executed by using the fields in front and back, frames in front and back, and video signals of upper and lower horizontal scanning lines in accordance with the level of motion. The method described here is called a motion detection scan conversion. (For example, Japanese Laid-open Patent 20002-95581).
FIG. 12 shows the relations of movie, telecine video signal, and video signal after scan conversion. Generally, movie 1201 consists of 24 frames per second. In the case of a video signal system of which the number of frames per second is 30, the number of frames per second in video signal does not match the number of frames per second in movie 1201. Then, movie 1201 is converted into interlaced scanning video signal apparently same in structure as conventional video signal. In this way, movie 1201 is converted into video signal, which is called telecine video signal 1203.
Each frame 1207, 1208, 1209 of movie 1201 is, same as common video signal, converted into the field of odd horizontal scanning line (hereafter referred to as the first TV field) and the field of even horizontal scanning line (hereafter referred to as the first TV field) to become telecine video signal 1203. In this case, field 1210 formed of frame 1207 is re-inserted between the two fields formed of frame 1207 and the two fields formed of frame 1208. Also, the field formed of frame 1209 is re-inserted after the two fields formed of frame 1209. The re-inserted fields are called repeat fields. Such conversion is repeated to obtain telecine video signal 1203. Thus, 4 frames of movie 1201 become 5 frames of telecine video signal 1203.
In this way, movie 1201 of 24 frames per second turns into telecine video signal 1203 based on an interlaced scanning system of 30 frames per second. The telecine video signal 1203 includes a repeat field in which same field is repeated every 5 fields. A method of generating such telecine video signal is called 2–3 pull-down.
The pattern of 5-field period peculiar to telecine video signal 1203 is called a telecine pattern. The telecine pattern is represented every field by telecine pattern number 1202 of P1 to P5 in the order of time. Also, the telecine pattern number of the repeat field described above is defined as P1, followed by P2 to P5 in order, which is repeated in order from P1 to P5.
Next, a conventional method for converting telecine video signal 1203 into progressive scanning video signal is described as an example.
Telecine video signal 1203 is subjected to the following processing in accordance with telecine pattern number 1202 for the purpose of executing scan conversion.
One-field earlier telecine video signal 1204 corresponds to telecine video signal 1203 delayed by one field. Two-field earlier telecine video signal 1205 corresponds to telecine video signal 1203 delayed by two fields.
For P1 of telecine pattern number 1202, the field 1210 of telecine video signal 1203 and the field 1211 of one-field earlier telecine video signal 1204 are used. Since the field 1210 and field 1211 are signals formed of same frames 1207 of movie 1201, the two fields are synthesized to obtain the frame 1220 of progressive scanning video signal 1206. Similarly, for P3, field 1214 and field 1215 are used to form frame 1222, and for P5, field 1218 and field 1219 are used to form frame 1224. Also, for P2 of telecine pattern number 1202, the field 1212 of one-field earlier telecine video signal 1204 and the field 1213 of two-field earlier telecine video signal 1205 are used. Since the field 1212 and field 1213 are signals formed of same frame 1208 of movie 1201, the two fields are synthesized to obtain the frame 1221 of progressive video signal 1206. Similarly, for P4, field 1216 and field 1217 are used to form frame 1223.
In the scan conversion of the above telecine video signal, when a common video signal that moves every field is applied, there is a possibility that completely different video signals are combined to form one frame, and it may cause the picture deterioration such as appearance of double pictures.
Also, when one telecine video signal is followed by the input of another telecine video signal, the scan conversion based on the telecine pattern number of the initial telecine video signal is executed on the following another telecine video signal. As a result, picture deterioration takes place.
FIG. 13 shows how image deterioration takes place under the above mentioned circumstances.
In FIG. 13, movie 1301, telecine pattern number 1302, telecine video signal 1303, one-field earlier telecine video signal 1304, two-field earlier telecine video signal 1305, progressive scanning video signal 1306, fields 1310 to 1319, and frames 1320 to 1323 respectively correspond to the movie 1201, telecine pattern number 1202, telecine video signal 1203, one-field earlier telecine video signal 1204, two-field earlier telecine video signal 1205, progressive scanning video signal 1206, fields 1210 to 1219, and frames 1220 to 1223 in FIG. 12, and the detailed description is omitted. Telecine video signal 1303 input is converted into progressive scanning. Telecine video signal 1303 is a signal formed with the first telecine video signal 1331 and the second telecine video signal 1332 changed over at time. 1330 and joined with each other. As mentioned in the description of FIG. 12, since the conversion into progressive scanning is executed on the basis of telecine pattern number 1302, the video picture is normal up to the frame 1323 of progressive scanning video signal 1306. Since the telecine pattern number 1302 at the time 1333 is P5, frame 1324 is formed on the basis of field 1318 and field 1319. However, after the time 1330, the repeat field 1340 of second telecine video signal 1332 appears at the telecine number P4. Field 1318 is a field formed of frame 1309 of movie 1301, and field 1319 is a field formed of frame 1308 of movie 1301. Accordingly, frame 1324 formed becomes a picture synthesized with different frames of movie 1301. This may cause the deterioration of the picture.